1. Field of the Invention
The present invention is generally directed toward apparatus and methods for offloading a high-pressure gas, such as compressed natural gas, from a storage vessel and reducing the pressure thereof to levels more suitable for use by vehicles, generators, heating equipment, and the like, while ensuring that the delivered product remains in gaseous form.
2. Discussion of the Prior Art
In the United States, natural gas has typically been transported in pipelines, and the pressures for local distribution are usually 50 psi or less. Regional networks supplying those systems are typically 720 psi or less with long distance transmission lines being typically 720 psi to 1480 psi. There are a few lines accommodating pressures of up to about 2150 psi. This grid supplies most of the U.S. where gas distribution networks exist. Areas in the northeast, which typically rely on fuel oil for heating, and rural and western areas that have a low density population that do not have enough usage to support the development of a supply network, rely on propane, electricity, wood or fuel oil to provide home heating and other energy needs for processing applications, irrigation and other energy uses.
As the relative price relationships of these energy sources has changed, due to new sources of energy being found, the economic opportunities created by these shifts in the status quo have created all sorts of new energy opportunities. Since natural gas is, in most cases, the lowest cost and usually most convenient energy form, there are lots of new conversion opportunities. Where pipelines are available, their use is preferable, but many newer opportunities, such as natural gas produced in remote petroleum extraction operations, cannot benefit because they are not served by existing natural gas distribution sources. These non-traditional sources have two natural gas alternatives: either compressed natural gas (CNG) or liquefied natural gas (LNG). Each has its own set of advantages and challenges.
LNG may be transported under low-pressure, but cryogenic conditions. Complex and capital-intensive cryogenic refrigeration systems are needed to liquefy and transport the natural gas in this fashion. With respect to CNG, economical storage and transportation requires that the gas be under high pressure, typically several thousand psi, but at or near ambient temperatures. However, most practical uses for CNG require the gas to be delivered at much lower pressures, typically less than 100 psi. Reducing the pressure of CNG from storage to use conditions can be very challenging, as a large pressure drop may result in significant reductions in gas temperature and even condensation of at least a portion of the gas, which may be incompatible with certain handling equipment. Moreover, because many opportunities for using the CNG recovered in remote locations lie within those same remote locations, permanent gas-handling facilities to adequately process the CNG to useable conditions are generally uneconomical.